1. Field of the Invention
This invention relates generally to rolling mills, and is concerned in particular with an improvement in single strand finishing blocks of the type employed to roll rods, bars and other like products in a twist-free manner.
2. Description of the Prior Art
An example of a well known single strand finishing block is described in U.S. Pat. No. 4,537,055, the disclosure of which is herein incorporated by reference. In this type of finishing block, successive roll stands have oppositely inclined pairs of grooved cantilevered work rolls. The block is driven by a common drive connected by means of a gear type speed increaser to a pair of line shafts extending in parallel relationship to the rolling line. Successive roll pairs are alternatively connected by means of intermediate drive components to one or the other of the line shafts. The intermediate drive components include intermeshed gears which provide fixed interstand speed ratios designed to accommodate the increasing speed of the product as it is rolled through the block.
The cross section of a product exiting from a conventional finishing block normally will be within tolerances which are acceptable for some but not all purposes. For example, a properly rolled 5.5 mm round will have a tolerance at or slightly below the limit of .+-.0.15 mm as specified by ASTM-A29. Such products may be used "as is" for many applications, including for example wire mesh, etc. For other uses, however, such as for example cold heading, spring and valve steels, much tighter tolerances on the order of 1/4 ASTM are required. Such products are commonly referred to as "precision rounds". In the past, this level of precision has been achieved either by subjecting the product to a separate machining operation after the rolling operation has been completed, or by continuously rolling the product through additional separately driven so-called "sizing stands". Sizing stands are conventionally arranged successively to roll products in a round-round pass sequence, with reductions in each pass being relatively light, e.g., 3.0%-13.5% as compared with reductions on the order of 20% per stand taken during normal rolling.
The sizing stands can be arranged in a separately driven block located downstream from the finishing block, or they can be incorporated as part of the finishing block. Separately driven sizing stands add significantly to the overall cost of the mill, and in some cases this arrangement may be impractical due to physical space limitations. The incorporation of the sizing stands into the finishing block minimizes these drawbacks. However, in the past, the fixed interstand drive speed ratios which exist between the successive stands of conventional finishing blocks has presented a limitation on the extent to which integrally incorporated sizing stands can be utilized.
For example, if the last two stands of a ten stand finishing block are adapted to operate as sizing stands, they can normally size rounds having a particular diameter and travelling at a particular speed as they exit from the preceding eighth stand. Should the rolling schedule subsequently call for a larger round, the normal practice would be to "dummy" (render inoperative) one or more successive pairs of stands in the finishing block in order to obtain the desired larger product. However, because the last two stands are operating at the same constant speed, they cannot accept the larger slower moving product. Thus, they too must be dummied, making it impossible to size the larger product.